Multi-path Effect

In wireless communication, multi-path fading is a real threat to reliable communication. During wireless transmission, a signal travels from the transmitter to the receiver via a number of different paths, which is multi-path effect as exhibited in Fig. 2.  Point A represents the transmitter and point B the receiver. The sum of these signals at point B will result in what is called constructive or destructive interference.
   In the QPSK application, the multi-path and relative motion between the transmitter and receiver cause amplitude attenuation, Doppler frequency shift, and phase shift in the constellation data points, as seen in Fig. 1. The Doppler frequency shift and coarse carrier synchronization cause the QPSK constellation to rotate and thus, decoding of the data is a real challenge. In order to lock onto the correct QPSK constellation and correctly recover the transmitted data, coarse and fine carrier synchronization is required.

                     Fig. 1.                                                                                  Fig. 2.

         QPSK constellation grid.                                      Multi-path effect. Point A represents a
                                                                                     transmitter and point B the corresponding
                                                                                     receiver. During wireless communication
                                                                                     signals arriving at point B may take many
                                                                                     different paths.